Generally, a fabricated article made from a polymeric material may become statically charged, and the surface may attract and hold charged particles such as dust in the air. In some cases an article may become damaged and/or otherwise devalued by the adhesion of electrostatically charged species. In other cases, such as in the packaging of powders, the powder may adhere to the packaging material. In addition to electrostatic adhesion, the buildup of excessive static charge on articles may result in damage to the article and/or its surroundings due to sparks and other rapid electrostatic discharges.
To prevent adhesion of charged particles, various approaches for preventing surface static charge buildup have been disclosed. For example, an antistatic agent or an antistatic polymer may be applied to the surface, or diffuse from the bulk composition to the surface, of an article as a way of preventing build-up of static charge on an article. See, for example, JP02-28919B, JP 61-44646A, and JP 10-193495A.
Use of surface-applied antistatic agents may introduce potential contamination of materials that come in contact with the surface-applied antistatic agent. Other drawbacks include situations wherein the applied film is inadequate in mechanical properties and could be easily damaged, has poor water resistance, or becomes tacky due to absorption of water. Attempts have been made to provide an antistatic polymer layer not as a surface layer, but as an inner layer with another polymer layer on the surface of the material.
Alternatively, antistatic agents may be incorporated into the composition of a molded article. However, materials that come in contact with the composition may become contaminated due to bleeding of the antistatic agent out of the composition and/or the antistatic effect may deteriorate with time. Permanent, non-diffusing antistatic agents avoid many of these problems.
Films containing ionomers can also be used for antistatic purpose. Ionomers have solid-state properties characteristic of cross-linked polymers and melt-fabricability properties characteristic of uncrosslinked thermoplastic polymers (see, e.g., U.S. Pat. No. 3,262,272). Ionomers may be prepared from copolymers of ethylene with an unsaturated carboxylic acid and optionally a softening comonomer. At least one alkali metal, transition metal, or alkaline earth metal cation, such as lithium, sodium, potassium, magnesium, calcium, or zinc, or a combination of such cations, is used to neutralize some portion of the acidic groups in the copolymer.
Neutralized (for example, greater than about 65%) potassium ionomer compositions or a cesium ionomer composition have been reported to exhibit good antistatic (surface resistivity) properties (H. Tachino, et al. Polymer Journal, 1994, 26, 10, pp 1170-1178).
An antistatic resin used as a packaging material or a coating material is disclosed. The resin comprises a copolymer of ethylene with an unsaturated carboxylic acid of which part or the whole is in the form of an alkali metal salt (see, e.g., JP60-240704; JP61-163853).
As the percentage of neutralized acidic groups increases, melt flow of potassium ionomer compositions may be dramatically reduced. Depending on the ethylene copolymer, high neutralization levels may result in compositions that do not flow in the melt. Using acid copolymers with high melt flow indices (low molecular weight) prior to neutralization provides some ability to maintain melt flow at higher neutralization levels. Such acid copolymers are difficult to manufacture and process. High melt flow acid copolymers may also react with ion sources only with difficulty because of low mixing shear. Also, mechanical properties (such as brittleness, especially at low temperatures) may be negatively affected using very high melt flow acid copolymers.
Highly neutralized ionomers may be modified with organic acids that may provide improved properties while retaining melt flowability. For example, WO2004/050362 discloses an antistatic composition comprising a blend of at least one E/X/Y copolymer (E is ethylene, X is a C3-C8 α,β ethylenically unsaturated carboxylic acid, and Y is a softening comonomer), and one or more organic acids or salts thereof where the combined carboxylic acid moieties in all ingredients in the blend are at least partially neutralized by potassium.
It is desirable to develop ionomeric compositions that provide antistatic properties while retaining sufficient melt-fabricability or melt processing properties, without adding low molecular weight species or using base resins with extremely high melt index, to allow them to be processed as thermoplastic materials.